The correct step to be taken prior to prescribing metformin is:
B. Draw a serum creatinine to assess renal function.
Assessing renal function before prescribing metformin is important because metformin is primarily excreted by the kidneys. Impaired renal function can lead to the accumulation of metformin in the body, increasing the risk of lactic acidosis, a rare but serious side effect. Therefore, evaluating renal function helps ensure the safe use of metformin in patients.